The present invention relates to automated endpoint detection systems during chemical-mechanical polishing (commonly known in the art as "CMP") of an integrated circuit substrate. More particularly, the present invention relates to automated endpoint detection systems that employ radiation sources and detectors to direct and detect radiation through a polishing pad to test for residual metal on an integrated circuit substrate during CMP.
CMP typically involves mounting a semiconductor wafer face down on a holder, and rotating the wafer against a polishing pad mounted on a platen, which in turn is rotating or is in an orbital state. A slurry containing a chemical that chemically interacts with the facing wafer layer and an abrasive that physically removes that layer is flowed between the wafer and the polishing pad or on the pad near the wafer. This technique is commonly applied to planarize metallization layers in the semiconductor wafer.
An endpoint determination mechanism currently employed during CMP to determine a polishing endpoint of a wafer layer includes a motor current measuring mechanism, as disclosed in U.S. Pat. No. 5,308,438, which is incorporated herein by reference in its entirety for all purposes. Briefly, according to this patent, the endpoint determination mechanism includes a motor, which rotates the wafer against a polishing pad. The power required to maintain a set rotational speed in the rotating motor changes (sometimes significantly) when a layer is removed from the wafer surface and a new wafer layer is being polished. In other words, as a layer is removed from the wafer surface, the amount of current being drawn by the motor changes due to the change in the frictional force induced by the changing wafer surface. Thus, by measuring the change in the current being drawn by the motor during polishing, the CMP endpoint of a wafer layer may be determined.
Endpoint determination using the motor current measuring mechanism, however, provides an average signal and is therefore imprecise, i.e. the motor current is not sensitive to the full surface of the wafer area during the monitoring of the polishing process. FIG. 1 shows the surface of a semiconductor wafer 100 that may result when the endpoint for CMP is determined by employing a motor current. As shown in FIG. 1, a plurality of die (or integrated circuits) 102 are formed on the surface of wafer 100, which has residual metal regions 104 that are large enough, i.e. on the order of several square millimeters, to cover several of die 102. Residual metal regions 104 represent underpolished or not completely polished metallization regions of the wafer surface that may render one or more die inoperable.
What is therefore needed is an improved endpoint detection system for detecting residual metal on an integrated circuit substrate that more precisely determines the CMP endpoint than the conventional polishing techniques.